1. Field of the Invention
The present invention relates to a wide-angle lens which is used for an imaging lens in a camera employing an imaging device such as CCD or imaging tube, a film, or the like; and, more specifically, to a wide-angle lens used for a projection lens in a projection type TV employing a liquid-crystal panel.
2. Description of the Prior Art
Conventionally, wide-angle lenses have been used in projection lenses in projection type TVs in order to increase the size of their screens or reduce their depth. Such a wide-angle lens has been required to secure a long back focus for inserting a mirror or the like in the middle of its optical path. There has hence been employed a lens configuration comprising a front group having a negative refracting power and a rear group having a positive refracting power.
In the above-mentioned wide-angle lens, since its distortion becomes large, it is necessary to use an aspherical lens on its enlargement side so as to correct the distortion. In this case, since aspherical lenses made of glass are expensive, those made of plastic may be employed for cutting down the cost.
In particular, since the enlargement-side lens of the wide-angle lens generally becomes greater in size, an aspherical lens made of glass having a refracting power would increase its cost considerably. Therefore, the use of a plastic lens is effective in terms of cost.
On the other hand, aspherical lenses made of plastic are disadvantageous in that they are less resistant to temperature changes, whereby their focal positions may change or their imaging performances may deteriorate due to temperature changes. Consequently, when using aspherical lenses made of plastic, they are often designed such that their refracting power is not so high.
In order to solve such a problem, Japanese Unexamined Patent Publication No. 8-320433 discloses a technique in which an aspheric lens made of plastic is used for a wide-angle lens.
When a wider-angle lens is to be produced by means of the above-mentioned technique using an aspheric lens made of plastic, its focal length becomes shorter, thereby shortening the space into which a returning mirror, a color-combining optical part, or the like is inserted. Also, when a glass lens is inserted into the first lens group on the enlargement side in order to secure a back focus space, thereby reducing the power of the aspherical lens made of plastic, the ratio of refracting power of the glass lens becomes higher in the first lens group, whereby the glass lens may enhance its size so as to increase the cost.